1. Field of the Invention
This invention relates to a method for manufacturing a permanent-magnet excited rotor for a high speed electric motor, according to which method a support sleeve is mounted with a pre-tension around the rotor body.
“High speed” motor means here each motor having a maximum speed requiring a support sleeve to keep the rotor from breaking, more particularly to keep the magnets, which have very low tensile strength, from breaking.
In practice magnets can mostly resist forces of 50 to 70 kPa, while in high speed motors tensile forces higher than 100 kPa can exist at maximum speed.
For high speed permanent-magnet rotors, a support sleeve is also required to keep the magnets securely attached to the rotor body and maintain these magnets under compression under all operating speeds and temperatures of the rotor.
To accomplish this, the support sleeve must be mounted on the rotor with a specified minimum interference, which is usually done by thermal expansion of the sleeve.
When the play between the sleeve and the rotor body is too big, or when the sleeve material has a too low thermal or negative expansion coefficient, such as carbon fibre, the sleeve will not be able to exert a sufficient pre-tension to the rotor body.
2. Discussion of the Related Art
A combined thermal and mechanical method is disclosed in U.S. Pat. No. 5,568,681. A capsule tube is fixed to the rotor body by shrinkage fit. One of both parts formed by the capsule tube and the rotor body is first fixed in a vertical direction in relation to a counter-support, and the other part is released from a location above the first part so that it will fall within the fixed first part under the influence of gravity.
Another known method is to hydraulically expand the sleeve with for example oil, and then while the sleeve expanded, insert the rotor body into the sleeve and then remove the pressure.
This generally requires tight mechanical tolerances on sleeve and rotor body, and also that the rotor body should be considerably stiffer than the sleeve.
The tolerance of the modulus of elasticity of the sleeve is also important. The magnets must also be mounted on the rotor in a way so they do not move when the sleeve is mounted, otherwise they may move when rotating, causing a redistribution of mass in the rotor which in turn may severely affect the balancing of the rotor.
There is also a complication if the rotor body does not contain a central shaft, or can be made as one mechanical body.